I. Field of the Invention
The present invention generally relates to a crystallizing method of a ferroelectric film having Pb in a composition.
II. Related Art and Other Considerations
Since Pb in a film is likely to be drawn out in a crystallizing step in the ferroelectric film (including the Pb in accordance with the present invention), a cap layer is formed on the film surface. The Pb drawing out operation is restrained by a thermal processing operation so as to effect the crystallizing operation. The Pb drawing out operation in the film is further prevented effectively by the use of short hours' lamp heating in the thermal processing operation. Thus prepared, a delicate film can be realized with the crystal grain diameter being uniform. Typically, a halogen arc lamp having wide wavelengths from infrared rays to ultraviolet rays is used as a heating lamp. An oxygen gas used in the thermal processing atmosphere is cracked so as to cause atomic active oxygen for effectively preventing an oxygen deficiency in the film. The same effect can be obtained also with the use of the ozone gas. Also, a gas including Pb may be used in the crystallizing step.
A method is used of growing a film on a base plate at a temperature of approximately 200.degree. through 300.degree. C. by sputtering. A subsequent thermal processing operation at 600.degree. through 700.degree. C. changes the crystal construction of a PZT film or a PLZT film (which is a ferroelectric film material) to a perovskite type crystal construction.
Since the Pb in the PZT (PbZr.sub.x Ti.sub.1-x O.sub.3), PLZT ((Pb.sub.1-x La.sub.x) (Zr.sub.1-y Ti.sub.y) .sub.1-y /40.sub.3) film is higher at an evaporation pressure as compared with Zr, Ti, La, the evaporation of PbO into the thermal processing operation and the composition ratio of the Pb in the film (after the thermal processing operation) is reduced, so that (hereinafter referred to as "stoichiometrical") the perovskite type crystal construction having chemically stablest composition cannot be obtained.
A film including the Pb excessively in advance is formed and a ferroelectric film of stoichiometrical perovskite type crystal structure is obtained by the thermal processing.
PbO is required excessively by approximately 15 through 20 mol for stoichiometrical PZT or PLZT film for excessive Pb in the film in the use of the above described method. Pin holes are likely to be caused, thus making it difficult to form the delicate film, because the PbO is evaporated when the PZT or PLZT film having the PbO excessively like this is thermally processed.
PZT (PbZr.sub.x Ti.sub.1-x O.sub.3), PLZT ((Pb.sub.1-x La.sub.x) (Zr.sub.y Ti.sub.1-y).sub.1-x /40.sub.3) and so on are known as a ferroelectric film including the Pb in the composition. Films having a given composition including the Pb are accumulated on the base plate by the sputtering method or a sol gel method when this type of ferroelectric film is conventionally formed. Thereafter, the base plate is set in a vessel which in turn is put into an electric furnace where the thermal processing operation is effected in the temperatures of 550.degree. through 650.degree. C. for approximately a few hours in an inert gas. The accumulated films (noncrystalloid) are changed into the perovskite type crystal structure to have the ferroelectric property for forming a ferroelectric film.
As the conventional forming method effects the above described thermal processing operation with the use of the electric furnace, at least approximately thirty minutes' time is required to be taken so as to raise the temperatures of the above described base plate and vessel, and to keep them stable. The crystal nuclei 12 which exist in the accumulated film 11 are excessively grown as shown in FIG. 2 (b), with a problem that the crystal grains 13 become excessively large in the ferroelectric film 11' after the thermal processing operation. When the crystal grains 13 in the film have become excessively large, stresses cause cracks in the ferroelectric film 11' and the conductivity becomes too high to be applicable as the dielectric film of the semiconductor element.
The evaporation pressure of the Pb becomes the highest in the composition of the above described ferroelectric film. A lot of Pb components are evaporated in the PbO condition (in the drawing, the component after the accumulation and the component after the thermal processing are shown) from the film 11 into the above described long hours' thermal processing as shown in FIG. 3 (b). The excessive Pb components of 10 through 20 mol % have to be added in advance into the accumulated film 11, with a problem that pin holes are caused in the ferroelectric film 11' are caused by the evaporation of the above described PbO to damage the film quality.